1. Field of Invention
This invention relates to a mixer input circuit, and particularly to a mixer input circuit for converting signals from an unbalanced-type tuning section into balanced signals and inputting the signals to a mixer.
2. Description of the Related Art
In order to optimize noise figures for an input circuit for inputting to a mixer of a so-called UHF tuner, it is conventionally known to carry out balanced input to the mixer. However, a radio frequency (RF) tuning section of a conventional tuning circuit is of an unbalanced type.
FIG. 1, for instance, shows an example of the conventional mixer input circuit.
Referring to FIG. 1, an RF signal from an antenna is transmitted via an RF input terminal 51 to an RF amplifier 52 so as to be amplified. The amplified RF signal is then transmitted to a first input terminal of an imbalance-balance conversion circuit 53 made up of an inductance element such as a coil. The first input terminal is grounded via a trimmer capacitor 54 while the other input terminal is grounded. An output signal from a first output terminal of the imbalance-balance conversion circuit 53 is transmitted via a capacitor 55C and a coil 55L to a first input terminal of a mixer 61. An output signal from the other output terminal of the imbalance-balance conversion circuit 53 is transmitted via a capacitor 56C and a coil 56L to the other input terminal of the mixer 61. The pair of output terminals of the imbalance-balance conversion circuit 53 are connected to each other through a trimmer capacitor 58. An oscillation output signal is supplied from a local oscillator 62 to the mixer 61, so as to be frequency-converted into an intermediate frequency (IF) signal and then to be taken out from an output terminal 63.
FIG. 2 shows another example of the conventional mixer input circuit. Referring to FIG. 2, an RF signal obtained from an input terminal 51 via an RF amplifier 52 is transmitted via a tuning circuit section 65 made up of a capacitor and coils to an imbalance-balance conversion circuit 66 employing a so-called balun transformer. Similar to the arrangement shown in FIG. 1, the RF signal of this example is frequency-converted into an IF signal by a signal from a local oscillator 62, so as to be output from an output terminal 63.
Meanwhile, the conventional mixer input circuit as described above requires that the imbalance-balance conversion circuit employ a transformer, and thus is likely to generate inconvenience, such as, a rise in production costs due to an increase in the number of parts and deterioration of performance due to interference of the conversion coil.